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    SubjectRe: [PATCH v3 4/5] i3c: add i3cdev module to expose i3c dev in /dev
    On Mon, 24 Feb 2020 11:04:50 +0000
    Vitor Soares <Vitor.Soares@synopsys.com> wrote:

    > Hi Boris,
    >
    > From: Boris Brezillon <boris.brezillon@collabora.com>
    > Date: Fri, Feb 21, 2020 at 22:32:16
    >
    > > On Wed, 19 Feb 2020 01:20:42 +0100
    > > Vitor Soares <Vitor.Soares@synopsys.com> wrote:
    > >
    > > > +static int i3cdev_detach(struct device *dev, void *dummy)
    > > > +{
    > > > + struct i3cdev_data *i3cdev;
    > > > + struct i3c_device *i3c;
    > > > +
    > > > + if (dev->type == &i3c_masterdev_type)
    > > > + return 0;
    > > > +
    > > > + i3c = dev_to_i3cdev(dev);
    > > > +
    > > > + i3cdev = i3cdev_get_drvdata(i3c);
    > > > + if (!i3cdev)
    > > > + return 0;
    > > > +
    > > > + /* Prevent transfers while cdev removal */
    > > > + mutex_lock(&i3cdev->xfer_lock);
    > > > + cdev_del(&i3cdev->cdev);
    > >
    > > When cdev_del() returns there might be opened FDs pointing to your
    > > i3cdev [1] ...
    >
    > Yes, I know. I protected the driver part but I missed the
    > file->private_data.

    Not sure what you mean by protection, but if you meant locking, then
    it's not enough: you need to refcnt the struct if you want to prevent
    use-after-free situations.

    BTW, I had a closer look at the usbdev implementation, and maybe you
    should base yours on usb instead of i2c. They seem to register a cdev at
    module init time, and add a dev_t per device at device registration
    time. Not sure how they handle the userspace-driver vs kernel-driver
    concurrency, but maybe returning EACCES (or EBUSY) instead of
    detaching/attaching the i3cdev everytime a device is bound/unbound
    would be simpler.

    Also, I think Arnd was right, it'd be simpler if i3cdev support was
    integrated to the core (still left as a option so it can be disabled,
    but with a dedicated i3cdev field in i3c_device instead hijacking the
    driver private field).

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